Utilising Flyash Paste Backfill

Society is in transition to a policy of sustainability - a long term approach that emphasises the efficient use of resources. To continue in business and minimise the negative commercial impacts of Government policies and taxes, industry has to engage its stakeholders and develop technologies which deliver sustainability.

Three areas of response to negative community concerns include:

• Greenhouse gas mitigation;
  
• Environmental research to provide the supporting science in new and existing projects; and
  
• R&D to respond to negative community sentiments that can, and do, threaten access to coal reserves.

The research and demonstration of waste minimisation of power station fly ash carried out for this ACARP project has provided a significant breakthrough for the coal sector. Apart from technology issues, mining environmental and community issues can be addressed by this project.

Coal reserves in the Newcastle area are being sterilised due to the risks associated with post mining surface settlements. Selective backfill using paste fly ash, has the potential to increase the reserves accessible to mining by providing an economic and environmentally acceptable backfill material. However, there has been no large scale economically feasible use of paste backfill in coal mines anywhere in the world to date.

There is a unique opportunity in the Newcastle coal mining area to exploit the large quantities of fly ash available from the adjacent power stations to selectively backfill coal mines and open up large areas of previously sterilised coal reserves. While fly ash, combustion ashes and other mining wastes have been used for coal mine backfill, they have generally been limited in their application. Large volumes of fly ash have been used for backfill in South Africa, but like much of the other backfills, this has been as an hydraulic slurry, with large volumes of supernatant water associated with the settling of the materials.

Full tailings paste backfill is being used in hard rock mines, is proving to be a viable and economic alternative to other backfill methods. Paste backfill has the following advantages over an hydraulic backfill:

• Paste is an environmental friendlier backfill - it exudes virtually no water on placing and as a result requires less water management;
  
• Paste requires far less underground containment preparation than hydraulic backfill; and
  
• Paste backfill placement operations do not have to be continuous - it has no critical flow velocity and can be left stationary in the distribution system for some time.

Testwork carried out for the Bayswater fly ash disposal system has shown that a high density fly ash slurry can be pumped a considerable distance. This study set out to confirm the suitability of fly ash to form paste mixtures and to demonstrate how they can be used as a successful backfill.

Backfills are being used overseas for a number of purposes, including thick seam, bord and pillar and longwall mining operations. This study proposes to demonstrate, through a number of critical stages, how this experience can be adapted to the Australian conditions with the use of fly ash as the backfill material. It is intended that this study will lead to the general application of backfill using a range of materials, potentially including coal washery rejects.